Various types of needleless hypodermic injectors have been designed previously. These types have had a number of drawbacks such as being heavy, cumbersome and difficult to apply to regions of restricted access, a fact which limits their use in fields such as dentistry.
Many of the known devices are dependent on heavy springs, which are first pre-stressed and then trigger-released to drive a piston that applies pressure to a liquid in a chamber. The liquid is then ejected from the chamber through a restricted orifice. It has generally been recognized that these devices should initially produce a high pressure for a short time in order to create a high velocity jet which can penetrate the skin to a desired depth. This is followed by a lower velocity jet which injects the liquid through the pierced skin where it can disperse radially into the surrounding tissue without penetrating to a greater depth or cause further, unintentional, tissue damage. The velocity and pressure of the initial jet and the length of time it is applied determine the depth of penetration and how much pain is felt by the injection.
The known injectors have often used a double piston impactor arrangement in order to create the desired high initial pressure. A first piston is brought to a high velocity before impacting against a second piston to create a high initial impact pressure in that type of arrangement. The second piston applies this pressure to a liquid in a chamber so that the liquid is ejected through a small orifice in the chamber with an initial very high velocity. Other injectors have used a hydraulic fluid in a cylinder to force a piston against a spring so the spring is prestressed. A trigger mechanism is then used to activate a valve arrangement which initially releases a relatively large amount of hydraulic fluid from the cylinder for a short time followed by a slower rate of release of the hydraulic fluid from the cylinder. That spring driven piston is connected to a further piston which applies pressure to a liquid in another cylinder in order to eject that liquid from that cylinder through a small orifice. The relatively large amount of hydraulic fluid initially released provides a high initial velocity to the pistons followed by a slower movement during the slower release of the hydraulic fluid. Both types of devices create a loud noise during their operation which, in the first type is due to the first piston impacting against the second piston. The second type creates a loud noise when the relatively large amount of hydraulic fluid initially released is stopped and changed to a slower release of the fluid. This noise can frighten or distract a patient being injected. Furthermore, a mechanical blow is delivered to the patient by the apparatus which is in contact with the skin when the pistons reach the end of their stroke. This results in some physical shock.
U.S. Pat. No. 3,292,621 describes one type of jet injector inoculator which is operated with a CO.sub.2 cartridge. This injector is designed to be operated with a particular type of ampule which is filled with inoculant, the ampule having a closure wall terminating in a nipple with a small orifice at one end The other end of the ampule is closed by a rubber piston. This ampule is inserted into and held in an opening at one end of the injector. The injector contains a piston in a cylinder with a ram extending from one side of the piston, the ram being aligned with the axis of the cylindrical ampule and having an end which is slightly spaced from the rubber piston in the ampule. A valve arrangement is used to apply CO.sub.2 pressure against the piston which initially does not move due to the end of the ram being held with "a snap ring. The snap ring has a circular cross-section and is located in a groove in a wall adjacent to the end of the ram. As the CO.sub.2 pressure builds up against the piston and reaches a predetermined level, the ram forces the ring outwardly, radially beyond the surface of the ram, and further into the groove. This releases the ram which then impacts on the rubber piston creating a high initial pressure in the ampule to eject the inoculant from the orifice at a high initial velocity. This is followed by the ram being pushed further into the ampule by the expanding gas behind the piston with the inoculant being ejected at a much lower velocity. This jet type inoculator has the same, previously mentioned, disadvantages as well as requiring a particular type of ampule.
U.S. Pat. No. 4,059,107 describes another type of injector with a first piston fitted into a first cylinder at one end of its housing. A rod extends between the first piston and a second piston which fits into a second cylinder at the other end of the housing. The second cylinder forms a liquid medication containing chamber which has a smaller diameter than the first cylinder to which it is joined by a stepped portion. A spring is located in the first cylinder between the piston and one end of the housing. Oil is inserted, at high pressure, through a check valve into the first cylinder on the side of the first piston opposite to the spring to prepare this injector. This forces the first piston towards the one end against the pressure created by the spring, which pre-stresses the spring. It also moves the second piston in the same direction creating a vacuum in the liquid medication chamber, which draws liquid medication from a vial, through a check valve, into the chamber. A cap with a nozzle having a small hole closes an end of the chamber and a check valve between the hole and chamber prevents air being drawn into the chamber when it is being filled.
A duct with a check valve is connected to the part of the first cylinder containing the oil. This duct leads to a third cylinder in which a third piston is spring-biased towards the end of the third cylinder connected to the duct. The third piston contains a small opening. A trigger mechanism is connected to the check valve in the duct and, when activated, opens that check valve to allow oil from the first cylinder to flow into the third cylinder, which forces the third piston towards the opposite end of the third cylinder.
This allows the pre-stressed spring to impart a high initial velocity to the first piston as the oil pressure in the first cylinder falls quickly until the third cylinder is full of oil. The third piston contains a small opening, and when the third cylinder is full, the piston being at opposite end of the third cylinder, oil flows through that opening. At that point, oil flows out through the opening in the third piston so that the oil pressure in the first cylinder falls at a slower rate resulting in a slower motion of the first piston. The interconnection of the first and second pistons creates the desired high initial pressure and ejecting velocity for the medication, followed by a lower ejecting velocity.
The same previously mentioned disadvantages exist with this type of design. A loud noise is created during operation, in this case by the third piston reaching the opposite end of the third cylinder, which stops the initial rapid fall of oil pressure in the first cylinder. This patent also suggests several alternative designs that can create a high initial pressure for the medication to pierce the skin followed by the injection of the medication through the pierced skin at a lower pressure. Both U.S. Pat. Nos. 4,059,107 and 3,292,621 are directed to portable hand held injectors.
U.S. Pat. No. 3,424,154 describes another type of apparatus for jet injection of fluids into soft tissues of a living body, in which a hand-held injection portion is connected via a flexible tube to a remote pneumatic motor. The hand-held portion consists of a probe terminating in a minute orifice from which a high-velocity jet of liquid may be ejected by movement of a piston inside the hand-held portion. The piston is activated by a liquid column in the flexible tube, which is connected to the pneumatic motor. The pneumatic motor includes a first piston and an auxiliary piston in a cylinder, the auxiliary piston being spring-biased away from the first piston, but moveable under the influence of fluid pressure to strike a hammer blow against the first piston. The fluid pressure then moves both pistons, and a further piston, which is connected to the first piston and contacts the liquid column, in a direction which will apply pressure to the liquid column. This will move the piston in the hand-held portion to create a jet of liquid from the probe. This apparatus, due to the auxiliary piston impacting against the first piston, emits a very high initial velocity jet followed by a much lower velocity jet. The apparatus described in U.S. Pat. No. 3,424,154 has a number of advantages over previous devices in that the pneumatic motor can be screened from the patient and the stopping of its pistons at the end of their stroke will not deliver a physical shock to the hand-held portion.
All of the previously mentioned devices have the limitation that the initial pressure and the pressure creating the lower velocity jet are determined by the dimensions and characteristics of the apparatus, and as a result are not readily adjustable. It is difficult to adjust the pressures of these devices to suit various skin conditions. For example, it may be possible to set a pressure which will painlessly penetrate the skin of a human or animal, but it is difficult to limit the depth of penetration and at the same time control the rate and volume of discharge. These difficulties may cause unintentional tissue damage and trauma.